A typical large-scale PV power plant employs a plurality of PV modules arranged in an array and DC/AC inverters and AC voltage step-up transformers located within the array for converting DC power from the PV modules to AC power for transport from the array to and connection with the utility grid. There are various ways to connect the PV modules through to DC/AC inverters to the grid which may include DC combiners for combining the DC outputs of the modules together and feeding the combined DC output from the modules to the DC/AC inverters located within the array. The transformers within the array step-up the AC voltage and the AC output from the transformers is then fed through a medium voltage AC (MVAC) power collection system to a substation located at a point of interconnection (POI) with the grid.
One such commonly employed system is shown in FIG 1. It includes groups of PV modules 11, with each group including a plurality of PV modules 11 with outputs connected in series, parallel or series/parallel relationship. Each module group is connected to a combiner box 15, sometimes referred to as a junction box, which electrically combines the DC outputs of the PV modules 11 of a group, or the DC outputs of one or more groups, together and sends the combined DC voltage over DC bus lines 17 to DC/AC inverters 19. The inverters 19 convert the DC outputs from the DC combiner boxes 15 to an AC voltage which is then sent to the step-up transformers 21, associated with each of the inverters 19, for a stepped-up medium voltage AC, e.g. 34.5 kV output voltage, which is applied to an AC medium voltage bus 23. The inverters 19 and associated transformers 21 form a power conversion station 100. The AC voltage outputs from the transformers 21 may also optionally be connected together in combining switchgear 38 and the combined AC voltage applied to the medium voltage AC bus 23. The AC bus 23 may connect the transformers 21 directly, or through the combining switchgear 38, to a power substation 25 located adjacent a point of interconnection with the utility grid 27. Although FIG 1 illustrates only four groups of modules 11 and combiner boxes 15 and two inverters 19 and associated transformers 21 and an optional combining switchgear 38, it should be appreciated that a large scale power plant will have many more PV module 11 groups, combiner boxes 15, inverters 19, associate transformers 21 and optional switchgear 38, arranged throughout an array, as illustrated in greater detail in FIGS. 3 and 4. For sake of simplicity, FIGS. 3 and 4 do not show the optional combining switchgear 38.
FIG 1 also illustrates in simplified form an example of a power plant control structure. It includes a power plant controller 60 located at the substation 25, or at another centralized location, and an associated SCADA user interface 34 for entering commands and data into power plant controller 60. The power plant controller 60 is also connected to the point of interconnection to the grid, via lines 36, for receiving measured grid parameters such as voltage, current and frequency. The power plant controller 60 sends commands to and receives data from a local controller 42, which in turn, controls the inverters 19 and the combining switchgear 38, if present. The local controller 42 sends commands to and receives data from power conversion station 100 and exchanges data with plant controller 60.
FIG 2 illustrates an enlarged view of a portion of FIG 1 with additional array structures, which may also be employed in the FIG 1 system. A back-up power supply 71 can be incorporated at the location of one or more inverters 19 which is connected to the output of inverter 19 and contains a step down transformer 73, an AC/DC converter 75, a DC storage system 81 e.g. a battery storage system, and a DC/AC converter 79. The DC/AC converter 79 supplies three phase AC voltage in the array as a supply voltage for a plurality of tracker controllers 16 which supply voltage and control signals to respective tracker actuators 44, which may be in the form of motors and associated linkages. The tracker actuators 44 operate to rotate a set of the PV modules 11 (also referred as table of modules) to track the position of the sun, or to place the PV modules 11 in a particular position for cleaning, maintenance or for protection of the PV modules 11 during storm conditions, as known in the art.
FIG 3 illustrates a top down view of the PV module array 32 layout of the FIGS. 1 and 2 system, while FIG 4 represents a partial enlarged and simplified electrical view of the array layout showing the interconnection of various elements. As can be seen in FIGS. 3 and 4, a plurality of DC lines 17 typically run in underground DC trenches 22 throughout the PV module array 32. The PV modules 11 are electrically connected to the combiner boxes 15 by lines 13 (FIG 4), while several combiner boxes 15 are, in turn, electrically connected to an associated inverter 19 by DC lines 17. The DC lines 17 typically run through the array 32 in a first direction shown by double-headed arrow A. The inverter 19 is connected to an associated transformer 21 (only one inverter 19 and transformer 21 are shown in FIG 4). The AC output of transformers 21, at for example, a medium three phase AC voltage of, e.g. 34.5 kv AC, is connect to AC bus lines 23 which typically run in underground trenches 24 through the array in a second direction indicated by double-headed arrow B, which is orthogonal to the direction A of the DC lines 17. Portions of the AC bus lines 23 are also routed through trenches 22 in a direction A into a substation 25 at a location adjacent the point of interconnection with the grid 27, as best shown in FIG 3. The AC voltage, which supplies the tracker controllers 16 (FIGS. 2, 4), is applied over AC wires 20 which may also be provided in the trenches 22.
The system illustrated in FIGS. 1-4 has several drawbacks. Typically tens of wires 13 at 1-2 kv DC from PV modules 11 are tied together in the combiner boxes 15 into a reduced set of +Ve, −Ve and ground wires 17, typically hundreds of feet long which are laid in the trenches 22. The DC wires 17 connect to the inverters 19 with fewer, thicker wires, typically at 1-2 kv DC. The running of the many DC wires 17 to the inverters 19 and associated transformers 21 within the PV module 11 array is cost intensive, both in terms of material and labor, and incurs power losses as well. In addition, the many inverters 19 and transformers 21 provided within and throughout the array, and the combining switchgear 38, if provided, and the wiring of the same is also material and labor cost intensive. The number of inverters 19 and transformers 21 which are provided throughout the array 32 also occupy valuable real estate, which could otherwise be occupied with additional PV modules 11. The AC lines 23 routed through the array in trenches 24 and 22 also add to material and labor costs. In addition, the AC lines 23 also must extend over distances which are typically thousands of feet long from transformers 21 to substation 25 which causes significant active and reactive power losses which, in turn, requires a larger electrical rating for the inverters 19 and transformers 21. Such line lengths and the number of lines 23 for a given size of array 32 also introduce transient voltage conditions on AC bus lines 23, e.g. when transformers 21 are energized, to the substation 25 which may cause transient conditions on the grid 27 and some delay in restarting the inverters 19 and transformers 21 after a grid failure.
At night time, when power is not being generated by the PV modules 11, the AC lines 23 between the transformers 19 and substation 25 are supplied with AC power by the substation 25 to keep transformers 21 energized to facilitate restarting the system for daytime operation with attendant no-load transformer 21 and AC line 23 losses. The no-load losses can be eliminated in an alternate scheme in which transformers 21 must be supplied with disconnect switches, which disconnect the transformers 21 from the AC bus lines 23 during night time conditions. Use of such a switch at each of transformers 21 adds to cost, and when such switches reconnect the transformers 21 to the AC bus lines 23, cause an undesirable high voltage transient condition on the grid, and loss of transformer mechanical integrity and therefore life, due to repeated cycles of magnetizing inrush current. The array 32 needs a utility voltage (AC grid voltage) for the in-array transformers 21, and inverter 19 to start, and practically cannot work in a stand-alone, so-called “island” operation mode, to supply power to a large load of similar rating in a situation where the array 32 is not connected to a grid. The lack of island mode is due to the practical challenges of synchronization (phase locking during startup and transient) of hundreds of inverters 19, in order to create a source of sufficient short circuit capacity for a large scale power plant when there is no grid voltage available. Due to the same challenge, the array 32 cannot establish a grid voltage at the utility grid for so-called “black start” conditions, where the utility needs voltage from the array to restart its grid through auxiliary generators.
Additionally, due to the presence of long medium voltage AC cables 23, the control bandwidth of inverters 19 to perform grid voltage support function is lower compared to the control bandwidth of a STATCOM (Static Compensator) device, well known in the art, to perform the same function. Overall, the inverters 19 and associated architecture generally has poor behavior in weak grids and relative slow response to transient grid conditions, resulting in significant integration challenges to integrate hundreds of inverters with the power system (grid). Furthermore, overall plant control latency is significant as the coordinated startup and operation of hundreds of inverters 19 in a large array takes time to execute.
An improved array architecture for mitigating many of these issues would be desirable.